1. Field of the Invention
The present invention relates to a display device and a driving method therefor. More particularly, the present invention relates to a display device for performing display by synchronizing a video signal with light source brightness control in controlling brightness of a light source according to the video signal.
2. Description of the Related Art
In recent years, non-emissive display devices as typified by liquid crystal display devices have been in widespread use. The non-emissive display device uses a light source and a light modulation device for adjusting the amount of light to be transmitted from the light source. The non-emissive display device has such disadvantages that the light source consumes power and that light leakage from the light source during the black display causes deteriorated contrast. In view of this, in order to make improvement in power consumption and contrast, there have been proposed a plurality of methods of controlling light source brightness according to video signals. As one example of such methods, JP 2008-076755 A proposes an image display device for controlling the light source brightness according to a histogram in which, for each predetermined gray-scale level, gray-scale levels representing the respective level-ranges in an input image are associated with frequencies of pixels included in the respective level-ranges.
Further, in general, the liquid crystal display device is a hold-type display device in which a rendered image is held until an image of the next frame is rendered, instead of an impulse-type display device, such as a display device using a cathode ray tube (CRT), in which strong light is emitted at the moment when an image is rendered. One of the drawbacks of the hold-type display device is so-called motion blur, in which an edge portion of a moving object becomes blurred when a moving picture is displayed. This motion blur is caused by so-called retinal persistence of vision, which occurs when a viewer moves his/her direction of sight along with the movement of an object, interpolating display images before and after the movement with regard to a display image for which the brightness is held. As a solution to such motion blur, a method of making the hold-type display device operate more like the impulse-type display device is known to be effective, such as updating display images with higher frequencies or temporarily canceling the retinal persistence of vision with insertion of a black screen (see “Moving Picture Quality Improvement for Hold-type AM-LCDs”, Taiichiro kurita, SID 01 DIGEST).
As one example of the methods for solving the motion blur, JP 2005-241787 A discloses a system (interpolation frame inserting system) in which an intermediate frame of two adjacent frames is generated as an interpolation frame based on the two adjacent frames, the interpolation frame thus generated is inserted between the two adjacent frames, and a video signal into which the interpolation frame is inserted is output with a higher frame frequency than the frame frequency of the original video signal.
As another example of the methods for solving the motion blur, JP 2006-343706 A discloses a system (light-dark field dividing system) in which a one-frame interval is divided into two field intervals for a light field and a dark field, display of the original video signal is performed using the two fields in a simulated manner, and the retinal persistence of vision is canceled by the dark field.
In both the interpolation frame inserting system and the light-dark field dividing system described above, the frame frequency of the video signal to be output needs to be converted into a higher frame frequency than the frame frequency of the input video signal (frame rate conversion). When the frame rate conversion is performed, a frame memory is used to temporarily hold the video signal.
Description is given of a problem posed when the brightness of the light source is controlled according to the video signal. FIG. 1A is a diagram for describing a schematic configuration of a liquid crystal display device related to a display device of the present invention. In particular, FIG. 1A is one example of a block configuration diagram of a display device including a mechanism for controlling light source brightness according to the video signal. The display device illustrated in FIG. 1A includes a display image data correction unit 108, a histogram generator 106, a backlight brightness calculation unit 107, and an image display unit 109. The image display unit 109 includes a liquid crystal panel 110 serving as a light modulation device and a backlight 111 provided behind the liquid crystal panel, which serves as a light source unit. Specifically, in the liquid crystal display device illustrated in FIG. 1A, the mechanism for controlling the light source brightness is independent of other video processing circuits, and also, an output signal from another video processing circuits is used as a video signal 102 to be input to the mechanism.
Next, by taking a case in which the video signal is input to the block configuration diagram illustrated in FIG. 1A at timing illustrated in a timing chart of FIG. 1B, focus is given to image data D0 corresponding to one frame of the video signal 102, and a problem therewith is described. For example, in a case of a high-definition TV format of 1,920 dots×1,080 lines with each color of RGB having 8 bits (total of 24 bits), the image data corresponding to one frame represents data constructed of 1,920×1,080×24 bits. The image data D0 is input to the histogram generator 106 and the display image data correction unit 108. The histogram generator 106 generates a histogram 104 showing a feature amount of the image data D0, which is used by the backlight brightness calculation unit 107 as an indicator in calculating backlight brightness based on the image data D0. When the histogram 104 is generated based on image data corresponding to one frame, as illustrated in the timing chart of FIG. 1B, data HG0, which is the histogram 104 of the image data D0, is generated with a delay of a one-frame interval T, compared to the image data D0. The histogram 104 generated as the data HG0 is output to the backlight brightness calculation unit 107. The backlight brightness calculation unit 107 uses the data HG0, which is the histogram 104 obtained by the histogram generator 106, to calculate light emission brightness of the backlight, and then outputs data BL0 serving as a backlight control signal 105 to the display image data correction unit 108 and the backlight 111. Here, focus is given to timings at which the image data and the backlight control signal 105 are input to the display image data correction unit 108. There occurs a time lag of the one-frame interval T between the timing at which the image data D0 is input to the display image data correction unit 108 and the timing at which the data BL0 is input to the display image data correction unit 108. Here, the data BL0 is the backlight control signal 105 obtained based on the data HG0, which is the histogram 104 generated based on the image data D0. As a result, there occurs a problem that the display image data correction unit 108 corrects the image data D1 by using the data BL0, which is the backlight control signal 105 obtained based on the image data D0, and then generates data D1′ serving as display image data 103.
As a method for solving this problem, JP 2008-076755 A proposes a method of using a frame memory. FIG. 2A is an exemplary block configuration diagram of a liquid crystal display device obtained by applying the method of using a frame memory to the liquid crystal display device illustrated in FIG. 1A. A frame memory 203 delays the image data D0 by the one-frame interval T, to thereby synchronize the image data D0 to be input to the display image data correction unit 108 with the data BL0, which is the backlight control signal 105 obtained based on the image data D0, before the image data D0 and the data BL0 are input to the display image data correction unit 108. Therefore, the display image data correction unit 108 can correct the image data D0 by using the data BL0, which is the backlight control signal 105 obtained based on the image data D0. However, in order to synchronize the image data with the backlight control signal 105, a frame memory is required, which prevents reduction in cost of the display device.